Thermal Fatigue Resistance of a Carbon-Carbon Composite
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THERMAL FATIGUE RESISTANCE OF A CARBON-CARBON COMPOSITE Q.C.GU, T.K.LEPISTO AND P.O.KETrUNEN Institute of Materials Science, Tampere University of Technology, P.O. Box 589, SF-33101 Tampere, Finland
ABSTRACT Thermal fatigue resistance of carbon-carbon composites with three different fiber surface treatments is studied in thermal cycles between 100 0 C and 1,700 0 C up to a number of cycles of 100 in free and restricted expansion conditions. The effects of thermal cycles were studied by SEM paying attention especially to structural damage and interfacial debonding between fibers and matrix. Bending tests subsequent to 10 thermal cycles were used to study the effect of thermal cycling on mechanical properties of the composites. The effect depends on the surface treatment of fibers prior to pyrolization. In some cases, the bending strength decreased due to the thermal cycling, whereas a suitable surface treatment minimized the damaging effect and increased the pseudo-ductility of the composite.
INTRODUCTION Carbon-carbon composite materials are now used in several high-tech components where exceptional properties are needed to ensure the reliable operation of the construction. Typical examples of their applications are different critical components in gas turbines, nozzles in airplanes and nose cones and wing leading edges in space shuttles [1] where resistance against rapid cooling and heating are provided from the materials used in them. For example, the temperature of the nose cone and leading edges of a space shuttle changes more than 1,800 oC during the re-entry thus forming favourable conditions for thermal fatigue to occur. Several methods are used to test the thermal fatigue resistance of materials, e.g. rapid cooling [2] and rapid heating [3]. In these tests the critical temperature difference AT to initiate the failure is used as the criterion for the thermal fatigue resistance of the material. Another approach is to measure the mechanical behaviour of the material after thermal shock tests. Although the thermal fatigue properties of several materials have been reported so far, only very few of them concern carbon- carbon composites. The present investigation tries to improve the situation by giving additional information on the subject.
MATERIALS AND EXPERIMENTAL PROCEDURES A carbon-carbon composite production technique, capable of producing an amorphous Mat. Res. Soc. Symp. Proc. Vol. 270. @1992 Materials Research Society
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matrix, was used in the present study [4]. The carbon fibers had the surface conditions: untreated (UTCF), oxidized (TCF) and provided with a size layer (STCF). The corresponding composites, UTCFC/C, TCFC/C and STCFC/C, were subjected to thermal shocks of a temperature change from 100 oC to 1,700 oC under free and restricted expansion conditions. Under the free expansion condition the specimens were placed freely in a graphite box connected to the water-cooled current leads. In the restricted expansion condition the specimens were directly connected to fixed, stationary current l
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